Wind turbines typically include a rotor with large blades driven by the wind. The blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy usually drives one or more generators to produce electrical power. Thus, wind turbines include a power transmission system to process and convert the rotational mechanical energy into electrical energy.
Oftentimes it is necessary to increase the rotational speed of the wind turbine rotor to the speed required by the generator(s). This is accomplished by a gearbox between the wind turbine rotor and generator. Thus, the gearbox forms part of the power transmission system and converts a low-speed, high-torque input from the wind turbine rotor into a lower-torque, higher-speed output for the generator.
Gearboxes typically comprises one or more planetary gear systems, which are also referred to as ‘epicyclic’ gear systems and the two terms are considered to be synonymous.
As is known, a planetary gear system includes a ring gear, a sun gear, and a carrier assembly including a plurality of planet gears that are rotatably mounted to a carrier. In one common configuration of planetary gear system, each one of the planet gears are rotatably mounted to a fixed shaft or pin that is itself received in a bore defined in the carrier. A bearing is interposed between the planet gear and the shaft and allows for smooth rotation of the planet gear. It is important that the shaft does not rotate in its bore, so the shaft is secured in the bore usually by way of a screw thread engagement, or by way of an interference fit. An interference fit may be achieved by differential thermal loading of the shaft and the bore prior to assembly. Both types of coupling techniques between the shaft and bore are known in planetary gear systems that experience high loading in use. For example, planetary gear systems or ‘sets’ are often used in gearboxes of utility scale wind turbine generators, particularly as a first stage in a multi-stage gearbox due to their relative compactness and suitability for the high torque loads generated by the rotor of the wind turbine generator.
However, high loading combined with the dynamic deflections caused by the wind turbine generator torque and by the vibration behaviour observed in planet gears can cause problems with the coupling between the shafts of the planet gears and the bores within which they are received. For example, in the case of a shaft that is secured in the bore by a bolt, the bolt is vulnerable to shear forces generated by high torque loading and dynamic deflections. This can cause failure of the bolt and, secondary damage, also to the planet gear. Alternatively, in the case where the shaft is engaged in the bore by an interference fit, the torque loading may cause the shaft to move angularly in the bore which may lead to severe wear or fretting corrosion of the shaft and/or the bore.
It is against this background that the invention has been devised.